Differential limiting control apparatus for four wheel drive vehicle

• US 6,189,643 B1
• Filed: 10/19/1998
• Issued: 02/20/2001
• Est. Priority Date: 10/24/1997
• Status: Expired due to Term

• Current Assignee: Subaru Corp.
• Original Assignee: Fuji Jukogyo KK (Subaru Corp.)
• Inventors: Takahashi, Akira, Hiwatashi, Yutaka
• Primary Examiner(s): Johnson, Brian L.
• Assistant Examiner(s): LUBY, MATTHEW D
• Application Number:
  US09/174,553
• Time in Patent Office: 855 Days
• Field of Search: 180/248, 180/249, 180/250, 701/88, 701/89
• US Class Current: 180/248
• CPC Class Codes: B60K 17/346 the transfer gear being a differential gear
• CPC Class Codes: B60K 17/348 having differential means for driving one set of wheels, e.g. the front, at one speed and the other set, e.g. the rear, at a different speed B60K17/346 takes precedence
• CPC Class Codes: B60K 23/04 for differential gearing
• CPC Class Codes: B60K 23/0808 for varying torque distribution between driven axles, e.g. by transfer clutch
• CPC Class Codes: B60K 28/165 acting on elements of the vehicle drive train other than the propulsion unit and brakes, e.g. transmission, clutch, differential acting on brakes B60T8/17
• CPC Class Codes: B60T 2201/14 Electronic locking-differential
• CPC Class Codes: B60T 8/1769 specially adapted for vehicles having more than one driven axle, e.g. four-wheel drive vehicles
• CPC Class Codes: B60T 8/322 Systems specially adapted for vehicles driven by more than one axle, e.g. Four Wheel-Drive vehicles
• CPC Class Codes: B60W 2520/125 Lateral acceleration
• CPC Class Codes: B60W 2520/14 Yaw
• CPC Class Codes: B60W 2540/12 Brake pedal position
• CPC Class Codes: B60W 2552/15 Road slope, i.e. the inclination of a road segment in the longitudinal direction
• CPC Class Codes: B60W 2552/40 Coefficient of friction
• CPC Class Codes: B60W 30/18145 Cornering

Abstract

When an accelerator pedal is released, or when a brake pedal is depressed and if braking force is lower than a reference braking force corresponding to a braking force immediately before causing a wheel lock, differential limiting force of a center differential is set to a predetermined value so as to retain an under-steer condition while engine brake is applied. When the brake pedal is depressed and if braking force is higher than the reference braking force, differential limiting force of the center differential is set to zero so as to assist an anti-lock brake system. Further, the reference braking force can be determined according to a road friction coefficient, a grade of road, a lateral acceleration and the like. Further, the predetermined value of the differential limiting force may be a constant value or may be a value corresponding to a front-to-rear weight distribution ratio.

Claims

1. A differential limiting control apparatus for a four-wheel drive vehicle having a center differential for distributing torque from an engine to the wheels of the vehicle, the control apparatus comprising:

accelerator release detecting means for detecting whether an accelerator is released,

braking force detecting means for detecting an amount of braking force applied to the wheels of the vehicle; and

differential limiting force adjustment means for adjusting a differential limiting force of the center differential,

wherein the differential limiting force adjustment means sets the differential limiting force to an initial differential limiting force reference value when the accelerator is released and the braking force is less than a braking force reference value, and

wherein the differential limiting force adjustment means sets the differential limiting force to zero when the accelerator is released and the braking force is greater than the braking force reference value.

2. The differential limiting control apparatus according to claim 1, wherein the braking force reference value is a fixed value.

3. The differential limiting control apparatus according to claim 1, further comprising:

an anti-lock brake system which, when activated, prevents sustained wheel-lock by controlling the braking force,

wherein, when the anti-lock brake control system is activated, the differential limiting force adjustment means adjusts the differential limiting force to an intermediate differential limiting force reference value which is less than the initial differential limiting force reference value but close to zero.

4. The differential limiting control apparatus according to claim 3,

wherein the anti-lock brake system includes wheel deceleration detection means, and

wherein the anti-lock brake system is activated when the deceleration of a wheel of the vehicle exceeds a specified deceleration value on braking.

5. The differential limiting control apparatus according to claim 4, further comprising:

means for adjusting a gear position into a higher range or a lower range,

wherein the means for adjusting the gear position shifts the gear into the higher range, when the anti-lock brake system is activated and the differential limiting force is adjusted to the intermediate differential limiting force reference value.

6. The differential limiting control apparatus according to claim 1, wherein the braking force reference value is a calculated value, the apparatus further comprising:

means for estimating a road friction coefficient; and

means for calculating the braking force reference value based on the estimated road friction coefficient.

7. The differential limiting control apparatus according to claim 6, further comprising:

an anti-lock brake system which, when activated, prevents sustained wheel-lock by controlling the braking force,

wherein, when the anti-lock brake control system is activated, the differential limiting force adjustment means adjusts the differential limiting force to an intermediate differential limiting force reference value which is less than the initial differential limiting force reference value but close to zero.

8. The differential limiting control apparatus according to claim 7,

wherein the anti-lock brake system includes wheel deceleration detection means, and

wherein the anti-lock brake system is activated when the deceleration of a wheel of the vehicle exceeds a specified deceleration value on braking.

9. The differential limiting control apparatus according to claim 8, further comprising:

means for adjusting a gear position into a higher range or a lower range,

wherein the means for adjusting the gear position shifts the gear into the higher range when the anti-lock brake system is activated and the differential limiting force is adjusted to the intermediate differential limiting force reference value.

10. The differential limiting control apparatus according to claim 6,

wherein the means for calculating the braking force reference value calculates a preliminary value based on the estimated road friction coefficient and compares this preliminary value with a predetermined upper limit value for the braking force, and sets the braking force reference value equal to the predetermined upper limit value when the preliminary value is larger than the predetermined upper limit value.

11. The differential limiting control apparatus according to claim 1, wherein the braking force reference value is a calculated value, the apparatus further comprising:

means for calculating road inclination; and

means for calculating the braking force reference value based on the calculated road inclination.

12. The differential limiting control apparatus according to claim 1, wherein the braking force reference value is a calculated value, the apparatus further comprising:

means for detecting a lateral acceleration being applied to the vehicle; and

means for calculating the braking force reference value based on the lateral acceleration being applied to the vehicle.

13. The differential limiting control apparatus according to claim 6, wherein the initial differential limiting force reference value is a calculated value, the apparatus further comprising:

means for determining engine brake torque based on an engine speed;

means for calculating engine braking force based on engine brake torque, a gear ratio at a current position, a final gear ratio and a tire radius; and

means for calculating the initial differential limiting force reference value based on a distribution of engine braking force between front and rear wheels corresponding to a front-to-rear weight distribution.

14. The differential limiting control apparatus according to claim 13, further comprising:

means for detecting wheel rotation speed; and

means for calculating wheel speed deceleration rate,

wherein the calculated initial differential limiting force reference value is corrected to a lesser value when the wheel deceleration rate is larger than a reference deceleration value.

15. The differential limiting control apparatus according to claim 14, wherein the reference deceleration value is calculated based on the braking force.

16. The differential limiting control apparatus according to claim 14, wherein the reference deceleration value is calculated based on the road friction coefficient.

17. A method of controlling a differential limiting force of a center differential of a four-wheel drive vehicle using the apparatus according to claim 1, the method comprising:

detecting whether the accelerator is released;

detecting the amount of braking force applied to the wheels of the vehicle; and

determining whether the calculated braking force is greater than a braking force reference value, wherein,

if the calculated braking force is less than the braking force reference value, setting the differential limiting force to an initial differential limiting force reference value, and,

if the calculated braking force is greater than the braking force reference value, setting the differential limiting force to zero.

18. The method according to claim 17, wherein the braking force reference value is a fixed value.

19. The method according to claim 17, further comprising:

preventing sustained wheel-lock by controlling the braking force with an anti-lock brake system; and

when the anti-lock brake control system is activated, adjusting the differential limiting force to an intermediate differential limiting force reference value which is less than the initial differential limiting force reference value but close to zero.

20. The method according to claim 19,

wherein the preventing sustained wheel-lock uses the anti-lock brake system which is activated when a deceleration of a wheel of the vehicle exceeds a specified deceleration value on braking.

21. The method according to claim 20, further comprising:

adjusting a gear position into a higher range when the anti-lock brake system is activated and the differential limiting force is adjusted to the intermediate differential limiting force reference value.

22. The method according to claim 17, wherein the braking force reference value is a calculated value, the method further comprising:

estimating a road friction coefficient; and

calculating the braking force reference value based on the estimated road friction coefficient.

23. The method according to claim 22, the method further comprising:

preventing sustained wheel-lock by controlling the braking force with an anti-lock brake system; and

when the anti-lock brake control system is activated, adjusting the differential limiting force to an intermediate differential limiting force reference value which is less than, the initial differential limiting force reference value but greater than zero.

24. The method according to claim 23,

wherein the preventing sustained wheel-lock uses the anti-lock brake system which is activated when a deceleration of a wheel of the vehicle exceeds a specified deceleration value on braking.

25. The method according to claim 24, further comprising:

adjusting a gear position into a higher range when the anti-lock brake system is activated and the differential limiting force is adjusted to the intermediate differential limiting force reference value.

26. The method according to claim 22,

wherein the calculating of the braking force reference value comprises calculating a preliminary value based on the estimated road friction coefficient and comparing this preliminary value with a predetermined upper limit value for the braking force, and setting the braking force reference value equal to the predetermined upper limit value when the preliminary value is larger than the predetermined upper limit value.

27. The method according to claim 17, wherein the braking force reference value is a calculated value, the method further comprising:

calculating road inclination; and

calculating the braking force reference value based on the calculated road inclination.

28. The method according to claim 17, wherein the braking force reference value is a calculated value, the method further comprising:

detecting a lateral acceleration being applied to the vehicle; and

calculating the braking force reference value based on the lateral acceleration being applied to the vehicle.

29. The method according to claim 21, wherein the initial differential limiting force reference value is a calculated value, the method further comprising:

determining an engine brake torque based on the engine speed;

calculating an engine braking force based on the engine brake torque, a current gear ratio, a final gear ratio and a tire radius; and

calculating the initial differential limiting force reference value based on a distribution of engine braking force between front and rear wheels corresponding to a front-to-rear weight distribution.

30. The method according to claim 29, further comprising:

detecting wheel rotation speed;

calculating wheel speed deceleration rate; and

correcting the calculated initial differential limiting force reference value to a lesser value if the wheel deceleration rate is larger than a reference deceleration value.

31. The method according to claim 30, wherein the reference deceleration value is calculated based on the braking force.

32. The method according to claim 30, wherein the reference deceleration value is calculated based on the road friction coefficient.

33. A method of controlling a differential limiting force of a four-wheel drive vehicle having a center differential for distributing torque from an engine to the wheels of the vehicle, the method comprising:

detecting whether an accelerator of the vehicle is released;

when the accelerator is released, calculating a braking force applied to the wheels; and

determining whether the calculated braking force is greater than a braking force reference value, wherein,

if the calculated braking force is less than the braking force reference value, setting the differential limiting force to an initial differential limiting force reference value, and,

if the calculated braking force is greater than the braking force reference value, setting the differential limiting force to zero.

34. The method according to claim 33, wherein the braking force reference value is a fixed value.

35. The method according to claim 33, further comprising:

preventing sustained wheel-lock by controlling the braking force using an anti-lock braking system; and

when the anti-lock brake control system is activated to prevent sustained wheel-lock, adjusting the differential limiting force to an intermediate differential limiting force reference value which is less than the initial differential limiting force reference value but close to zero.

36. The method according to claim 35,

wherein the anti-lock brake system includes wheel deceleration detection means, and wherein the anti-lock brake system is activated when the deceleration of a wheel of the vehicle exceeds a specified deceleration value on braking.

37. The method according to claim 36, further comprising:

adjusting a gear position into a higher range when the anti-lock brake system is activated and the differential limiting force is adjusted to the intermediate differential limiting force reference value.

38. The method according to claim 33, wherein the braking force reference value is a calculated value, the method further comprising:

estimating a road friction coefficient; and

calculating the braking force reference value based on the estimated road friction coefficient.

39. The method according to claim 38, further comprising:

preventing sustained wheel-lock by controlling the braking force using an anti-lock braking system; and

when the anti-lock brake control system is activated to prevent sustained wheel-lock, adjusting the differential limiting force to an intermediate differential limiting force reference value which is less than the initial differential limiting force reference value but close to zero.

40. The method according to claim 39,

wherein the anti-lock brake system includes wheel deceleration detection means, and

wherein the anti-lock brake system is activated when a deceleration of a wheel of the vehicle exceeds a specified deceleration value on braking.

41. The method according to claim 40, further comprising:

adjusting a gear position into a higher range when the anti-lock brake system is activated and the differential limiting force is adjusted to the intermediate differential limiting force reference value.

42. The method according to claim 38, wherein the calculating of the braking force reference value comprises:

calculating a preliminary value based on the estimated road friction coefficient, and

comparing the preliminary value with a predetermined upper limit value for the braking force; and

setting the braking force reference value equal to the predetermined upper limit value when the preliminary value is larger than the predetermined upper limit value.

43. The method according to claim 33, wherein the braking force reference value is a calculated value, the method further comprising:

calculating road inclination; and

calculating the braking force reference value based on the road inclination calculation.

44. The method according to claim 33, wherein the braking force reference value is a calculated value, the method further comprising:

detecting a lateral acceleration being applied to the vehicle; and

calculating the braking force reference value based on the lateral acceleration being applied to the vehicle.

45. The method according to claim 33, wherein the initial differential limiting force reference value is a calculated value, the method further comprising:

determining engine brake torque based on an engine speed;

calculating engine braking force based on the engine brake torque, a current gear ratio, a final gear ratio and a tire radius;

calculating the initial differential limiting force reference value based on a distribution of engine braking force between front and rear wheels corresponding to a front-to-rear weight distribution.

46. The method according to claim 44, further comprising:

detecting wheel rotation speed;

calculating wheel speed deceleration rate; and

correcting the calculated initial differential limiting force reference value to a lesser value when the wheel deceleration rate is larger than a reference deceleration value.

47. The method according to claim 46, wherein the reference deceleration value is calculated based on the braking force.

48. The method according to claim 46 wherein the reference deceleration value is calculated based on the road friction coefficient.